The throttle of an internal combustion engine is a valve that directly regulates the amount of air entering the engine, indirectly controlling the power output of the engine. At wide open throttle, the intake manifold is at or near ambient atmospheric pressure. When the throttle is partially closed, a manifold vacuum develops and the resistance to movement of the pistons in the cylinders increases.
An accelerator pedal is typically used by a vehicle operator to mechanically control the position of the throttle. In other vehicles the throttle is electronically-controlled based on engine operating conditions, such as the position of the accelerator pedal, but there is no mechanical linkage between the accelerator pedal and the throttle.